What happens if you log off the account from the webmail page? Is there another device (computer or phone) from which you access the account?

What do you mean by logging off th webmail account?

I can also access the account through my mobile phone.

I mean that you should deliberately Sign Out or Log Off the webmail account by clicking the relevant link, not just close the browser window. Same with the phone.

It didn't help. As a matter of fact, the mail stopped coming at lunchtime past Sunday and I didn't log on to the webmail until Monday night. As for connecting through the mobile phone, my old phone was stolen some days ago and I bot a new one on Sunday. The e-mailaccount was installed only on Monday evening... (Both the old phone and it's sim-card were immediately blocked after the theft.)

Have you asked your email provider what the message means? It is afer all your mail provider that has locked your account.

Yes I asked. That was the first thing I did. And there's nothing wrong on the provider's side. They didn't lock the account and found it very unusual. They didn't know much about Thunderbird, however...

But I got hands-on-help from my son yesterday and that might have (almost) solved it. Seems that Thunderbird can't have both IMAP and POP3 accounts running and there might have been something in this that disturbed the flow. He made a new inbox and things started to work again. But it's still not working 100 %.

I have multiple accounts, both POP and IMAP, and I have run the same account (gmail) in POP and IMAP simultaneously.

So I don't think there is an inherent limitation in Thunderbird, but your mail provider my object to you being logged in more than once at a time, and IMAP has a way of staying logged on all the time....so if you have both POP and IMAP connections to an account, the POP account may be be locked out by the IMAP connection, if your provider tolerates only one connection at a time.